The present invention relates to the production of copolymers of polyethylene naphthalate ("PEN") and polyethylene terephthalate ("PET"). Polyethylene naphthalate incorporates naphthalene's double aromatic ring structure which gives it certain enhanced characteristics as compared to polyethylene terephthalate which incorporates benzene's single-ring structure. In particular, the increased rigidity of the double-ring structure gives polyethylene naphthalate higher strength, greater heat stability, and improved barrier properties compared to other polyesters.
For these and other reasons polyethylene naphthalate is a particularly strong candidate for packaging materials. Polyethylene naphthalate has a higher glass transition temperature and a reduced gas permeability as compared to polyethylene terephthalate.
As an alternative to pure PEN, blends and copolymers of polyethylene terephthalate and polyethylene naphthalate offer packaging advantages over polyethylene terephthalate standing alone. In this regard, the oxygen-sensitivity and required packaging temperatures of particular foods tend to define the melt temperature and oxygen permeability of the packaging materials that can be used with those foods. Thus, polyethylene terephthalate can withstand packaging temperatures up to about 180.degree. F. and offers oxygen permeability as low as 5 grams per milliliter per atmosphere per day. By comparison, polyethylene naphthalate can withstand process temperatures of up to about 250.degree. F. and, for foods sensitive to oxygen exposure, its oxygen permeability approaches 0.5 grams per milliliter per atmosphere per day.
As a result, blends and copolymers of polyethylene terephthalate and polyethylene naphthalate can offer a number of the advantages of PEN, but at a lower cost than pure PEN (present pricing of raw materials, cost is a current disadvantage in the use of pure polyethylene naphthalate). Blends and copolymers of polyethylene terephthalate and polyethylene naphthalate can withstand packaging temperatures of 200-210.degree. F., and both offer lower oxygen permeability as compared to polyethylene terephthalate. For example, the copolymers offer oxygen permeability of about 4 grams per milliliter per atmosphere per day, while the blends offer oxygen permeability as low as about 1 gram per milliliter per atmosphere per day. As a result, items such as jellies, sports drinks, and tomato sauces that must be packaged at temperatures higher than polyethylene terephthalate can withstand can be packaged in PET-PEN copolymers. Other items requiring lower oxygen permeability than PET can offer, such as catsup and certain carbonated soft drinks, can be successfully packaged in the PET-PEN blends.
As known to those familiar with polyester and its method of manufacture, most polyethylene terephthalate is currently made in the "direct esterification" process. In the initial step, terephthalic acid and ethylene glycol undergo an esterification reaction to form an ester, with water as a byproduct. A corresponding direct esterification can be carried out using diacids of naphthalene. As a present disadvantage, however, the diacids of naphthalene tend to be somewhat less pure than terephthalic acid, and the impurities have undesired side effects on the reaction and the equipment used to carry it out.
The naphthalene diesters, however, are available in higher purity than their corresponding acids. The esters thus offer an attractive starting material for polyethylene naphthalate polymers and copolymers.
The esters nevertheless raise a disadvantage in that they produce alcohols, rather than water, as their byproduct in the initial esterification reaction. Because terephthalic acid produces water as its byproduct, many current facilities for polyester manufacture are well equipped to handle water as the byproduct. These facilities are not, however, as well equipped--or indeed not equipped at all--to handle alcohol byproducts. As those of ordinary skill in this art recognize, although some characteristics of water and lower alcohols are similar in the liquid and vapor phases, other characteristics are quite different. As a result, alcohols either should not or cannot be disposed of using all of the identical equipment or techniques that can be used to dispose of water.
In copending application Ser. No. 08/672,578, filed Jun. 28, 1996, for "Destroying 1,4-Dioxane in Byproduct Streams Formed During Polyester Synthesis," and now U.S. Pat. No. 5,817,910, which is commonly assigned with the present invention, a technique is disclosed in which the reaction mixture of the esterification of the polyester precursors (dihydroxyl alcohol and dicarboxylic acid) are distilled to remove at least a portion of the esterification byproducts from the reaction mixture in the form of a vapor stream. Although the majority component of the vapor stream tends to be water, it also contains other undesired byproducts that need to be disposed of properly.
The '578 application focused on dioxane removal and disclosed that sending the vapor stream to a combustion burner (rather than condensing it as in conventional processes), easily destroyed the dioxane before it was ever emitted as a byproduct or pollutant. The addition of the water vapor to the combustion burner lowers its efficiency somewhat, of course, but the production advantages gained by destroying the dioxane generally outweigh the small loss of efficiency suffered by the burner when the water vapor is added.